As oil continues to spew from the oil rig that went up in flames in the Gulf of Mexico last week, the probability of major
environmental damage increases. For now, frantic clean-up efforts and favorable winds are preventing 18 square kilometres
of thick oil slick and thousands more square kilometres of oily sheen from reaching the shore.

About 160,000 litres of oil per day are gushing from two holes in the pipe that once connected the 1.5-kilometre-deep well
to the Deepwater Horizon rig at the surface. How long the spill continues depends on which of several emergency efforts under
way is successful.

The most threatening portion of the spill is a black oil slick that runs east and west from the accident site and comes within
65 kilometres of marshy coast near Venice, Louisiana. If the wind switches from its current offshore direction the slick could
be driven into the marshland wreaking environmental havoc.

"Those marshes are very, very sensitive areas", says Nancy Kinner, an environmental engineer and co-director of the Coastal
Response Research Center at the University of New Hampshire in Durham. The marshes serve as nurseries for countless economically
and ecologically important fish and crustaceans whose young might be smothered or otherwise harmed, she says.

Cleaning up the marshes would be very difficult. Kinner says that oil washing onto sandy beaches elsewhere along the coast
would be a much better outcome because oily sand can be scraped up for disposal. "Everything is a trade-off," she says, "But
once the oil is out of the well something is going to be damaged."

Burn it clean

A response team to tackle the oil spill has been established, which includes the US Mineral Management Service, the National
Oceanic and Atmospheric Administration (NOAA), rig owner Transocean and BP, which had leased the rig. At a press conference
on 26 April, Rear Admiral Mary Landry of the US Coast Guard, who heads the response team, said that forecasts should allow
states around the gulf time to prepare for and stave off the worst of the damage if the oil approaches land.

“If we can turn the oil into smoke we'll all be happy.”

One way to clean up spilled oil in coastal marshes is to burn it. This method was used to deal with a spill after Hurricane
Katrina, and the marshes recovered quickly. Ed Levine, NOAA's scientific support coordinator for the effort, says that burning
the oil in open water before it reaches land is another possibility if the weather is calm enough. "If we can turn the oil
into smoke we'll all be happy," he says — although there are some concerns that the smoke may drift inland.

The oil slick also poses some threat to life while it is in the open ocean. The threat to fish and mammals is real but relatively
low as the oil is mainly confined to the surface — but it can still harm plankton and larvae there.

Scientists say that the much larger oily sheen, which will dissipate on its own, is less of a danger. But toxic chemicals
in this oil can also harm plankton and larvae, and may be more damaging closer to shore where mixing and containment in shallow
water increases exposure. "Quite frankly we don't know that much about these kinds of things," says Kinner. "It's very hard
to say what the true impact would be over a long period of time."

Spill bill

The response team is now working to shut off the flow of oil using the 400-tonne blow-out prevention device on the sea floor.
The team is using remotely operated vehicles to try to fully engage the device, which is only partially closed, and so halt
the flow completely.

A back-up effort about to get under way is drilling a new well into the old one to relieve pressure, allowing it to be capped.
This method was used to cap the Australian West Atlas oil rig spill last year, and took three months. Digging a relief in
the Gulf is also likely to take months.

Around 30–40% of the oil should evaporate over time. To contain the rest, the Gulf response team is skimming oil from the
surface and using planes to spray the main slick with dispersants, one litre of which can treat 20 to 30 litres of oil. Over
400,000 litres have been stockpiled, and BP has asked companies to increase their production. The team is also working to
deploy a dome-shaped containment device that can capture leaking oil and funnel it to the surface for disposal.

Once the response team staunches the main spill, officials will decide whether anything can be done with the rig, which sunk
with more than 2.5-million litres of diesel fuel. At present, there are no signs of fuel leakage.

No estimates are available for how much the overall operation will cost, but under US law BP is responsible for the complete
bill, including reimbursement for any damages from ecological and commercial losses if the oil hits the shore.